Pacemaker mediated tachycardia (PMT) is a problem in conventional pacemakers that operate in an atrial synchronous pacing mode (e.g., DDD). If the patient's heart has retrograde conduction pathways (i.e., from the ventricle to the atrium) and there is a ventricular event at the opportune time (e.g., a premature ventricular contraction), a PMT can be established. The ventricular event is conducted to the atrium where it is sensed by the atrial channel of the pacemaker as if it were an inherent atrial event, i.e., a P-wave. This sensed atrial event initiates an A-V Delay and a ventricular pacing pulse. The resulting ventricular depolarization (V-pace) is retrogradely conducted to the atrium, and so a repetitive cycle is established. If the retrograde conduction time is consistent, then the ventricular pacing rate is determined by the sum of the A-V Delay and the retrograde conduction time.
The earliest dual chamber pacemakers attempted to solve the problem of pacemaker mediated tachycardia by limiting the ventricular pacing rate to a predetermined maximum rate. Unfortunately, such a ventricular rate limitation precludes a patient from realizing the advantages of atrioventricular (A-V) synchrony under circumstances when an elevated heart rate and A-V synchrony would be most beneficial, such as when exercising. One prior art technique allowing A-V synchronous pacing at higher atrial rates is to extend the A-V intervals for a number of pacing cycles, during which the ventricular pacing rate is not allowed to exceed the maximum rate. If a ventricular pace is scheduled which would cause the ventricular rate to surpass the maximum rate, it is skipped and, in the next cardiac cycle, the pacemaker reverts to A-V synchronous operation. A disadvantage of this procedure is that it carries the risk of creating excessively long actual A-V interval times which may themselves lead to PMT, particularly when the maximum rate is set to a relatively low value.
A common method for preventing PMT in an A-V synchronous pacemaker is to lengthen the atrial refractory period, also called the post ventricular atrial refractory period (PVARP). A disadvantage of this procedure is that it reduces the upper heart rates to which the pacemaker may respond synchronously. This procedure may be modified so that the PVARP is extended only when the probability of a retrograde VA transition is high, such as, for example, following a premature ventricular contraction (PVC). A PVC is a ventricular heartbeat which does not follow an atrial event. Unfortunately this may lead to a continued failure of P-wave sensing. Furthermore, an extension of the PVARP does not guarantee that a PMT will be terminated.
Several procedures have been advanced for automatically adjusting PVARP durations to avoid PMT, but yet to maintain an adequate upper rate response in a dual chamber pacemaker. U.S. Pat. No. 5,102,820 for "VDD Pacemaker with Selectable Post-Ventricular Atrial Refractory Periods", issued Apr. 14, 1992 to H. T. Markowitz, and U.S. Pat. No. 5,123,412 for "Dual-Chamber Pacemaker with Automatic Selection of Atrial Refractory Periods", issued Apr. 14, 1992 to R. A. Betzold, disclose pacemakers which prevent PMT by setting the PVARP duration as a function of whether the ventricular pacing pulse is triggered by the expiration of the A-V interval or by the expiration of the lower rate interval. A relatively short PVARP (e.g., 250 ms) is triggered by expiration of the A-V interval. Expiration of the lower rate interval is followed by a longer PVARP (e.g., 400 ms). The shorter PVARP following an A-V interval expiration is not likely to induce retrograde conduction because it is closely preceded by an atrial contraction. The longer PVARP following lower rate timeout prevents ventricular pacing pulses, which are not triggered in response to sensed atrial contractions, from inducing PMTs.
U.S. Pat. No. 5,129,393, entitled "Dual Chamber Rate Responsive Pacemaker with Variable Refractory Period" and issued Jul. 14, 1992 to D. A. Brumwell, prevents PMT by setting PVARP duration as a function of both a physiological sensor-determined ventricular pacing rate and the natural atrial rhythm. The sensor-determined ventricular rate is intended to correlate to a patient's metabolic demand. At low metabolic demand levels and atrial rates, the PVARP is lengthened to prevent premature atrial contractions from triggering ventricular pacing pulses. Therefore, the pacemaker tracks physiologically-appropriate rapid atrial rates in the presence of a low metabolic demand by following a gradual increase in intrinsic atrial rate with a gradual decrease in PVARP duration. Conversely, the pacemaker responds to a high metabolic demand in combination with a low intrinsic atrial rate by shortening the PVARP, allowing ventricular synchrony with appropriately timed natural atrial contractions. When the PVARP duration is short is the most likely condition for triggering PMT. Therefore, the pacemaker should be programmed to assure that an average atrial rate based on retrograde P-waves is not effective to maintain PMT.
A method and apparatus for detecting PMT is taught in U.S. Pat. No. 4,569,350, entitled "System For Detecting Pacer Mediated Tachycardia", issued to V. E. Mumford et al. on Feb. 11, 1986. Atrial P-waves and ventricular R-waves are sensed. When a P-wave is sensed, after a predetermined A-V Delay, a stimulating pulse is provided to the ventricle if no R-wave is sensed during the A-V Delay. A threshold rate is selected for the sensed P-P interval. If the ventricle has been stimulated and the threshold rate is exceeded for a selected number of cardiac cycles, the A-V Delay is increased by a predetermined time, delta. A determination is then made whether the next P-P interval has increased by delta. If so, a phenomenon termed "A-V precession" or simply "precession", then this is an indication that PMT has occurred.
The previously discussed procedures of limiting ventricular pacing rate and adjusting PVARP durations operate to prevent PMT at the expense of maintaining A-V synchrony when a patient is experiencing a high metabolic demand. Another procedure for dealing with PMT, which provides for an improved upper rate response in a pacemaker, is to maintain a short PVARP duration but to detect and treat PMT when it occurs. For example, in U.S. Pat. No. 4,554,921, entitled "Dual Chamber Pacemaker With Automatic High Rate Limit Mode Determination" and issued Nov. 26, 1985, W. Boute et al. discloses a method for monitoring retrograde P-waves and avoiding pacemaker mediated tachycardia, if possible. If avoiding PMT is not possible, the pacemaker detects and terminates PMT. The Boute et al. pacemaker does this by monitoring V-A stability. The V-A interval may be indicative of the retrograde conduction time for propagating cardiac depolarization from the ventricles to the atria. From the V-A stability, the pacemaker detects retrograde P-waves and PMT. The pacemaker breaks up PMT by skipping a ventricular stimulus which would normally be delivered in synchronous response to a sensed P-wave.
Another procedure is to avoid inducing PMT when it is most likely to occur. When a pacemaker switches from a VVI to a DDD mode, there is a substantial risk that synchronization will start precisely at the moment an atrial depolarization is triggered by a retrograde transition, and thus will initiate a pacemaker mediated tachycardia. Therefore, U.S. Pat. No. 4,802,483, entitled "Heart Pacemaker For Avoiding Pacemaker Mediated Tachycardia at Mode Switching" issued Feb. 7, 1989 to A. Lindgren, discloses a pacemaker which is intended to switch between VVI and DDD modes without causing PMT, while retaining good upper rate operation. Lindgren does this by determining a heart condition appropriate for switching to the atrial synchronous mode, and then delaying the transition to the atrial synchronous mode until the second (or later) atrial signal following the ventricular event, so that the first atrial signal does not trigger stimulation in the atrium. Therefore, the second atrial signal cannot have been produced by the retrograde transition of a ventricular signal, but rather it is representative of a heartbeat of true atrial origin. If no atrial signals are forthcoming, the pacemaker waits a predetermined delay interval (e.g., 200 ms or a programmable duration) and begins pacing in A-V synchrony.
In U.S. Pat. No. 5,085,215, entitled "Metabolic Demand Driven Rate-Responsive Pacemaker" and issued on Feb. 4, 1992, T. A. Nappholz et al. describe a pacemaker that automatically selects an appropriate pacing mode, either DDDR or VVIR, based on an analysis of a patient's natural atrial rate and its relationship with a metabolic indicator rate derived from the output of a physiological sensor. This pacemaker compares the sensed natural atrial rate to a "maximum rate for synchronous pacing", which is a function of the derived metabolic indicator rate, to classify an atrial rhythm as physiological or pathological. The upper rate response mechanism of this pacemaker provides for A-V synchrony at natural sinus rates ranging from low rates to high exercise rates, even up to the programmed maximum rate, but maintains ventricular rate stability during pathological atrial tachycardias by reverting to pacing in a rate-responsive VVI mode.
A PMT may drive the atrial heart rate at varying speeds, including fast pathological rates and slower physiological rates that may arise when a patient exercises. The retrograde conduction times which give rise to a PMT may be constant or irregular. It is an objective of the present invention to detect, confirm and terminate PMT, whether the atrial rates are pathological or physiological, regardless of the regularity of retrograde conduction. (The invention tests for the PMT condition continuously, confirms PMT rarely, and executes the PMT termination procedure even less frequently.)